1. Field
The invention relates to applications for wireless communications. More particularly, the invention relates to a universal real-time interface for wireless modems.
2. Background
FIG. 1 is a simplified prior art system 100 showing protocol stacks for a transmitter 101 and a receiver 106. The two protocol stacks are based on the Open System Interconnection (OSI) Reference Model. The two protocol stacks are simplified examples for the transmitter 101 and the receiver 106. The simplified protocol stack for the transmitter 101 (listed top-down) includes an application layer 102, a transport layer 103, a data link layer 104, and a physical layer 105. The simplified protocol stack for the receiver 106 (listed top-down) includes an application layer 107, a transport layer 108, a data link layer 109, and a physical layer 110. The two physical layers 105 and 110 are connected to a wired-wireless network 113 and are configured to deliver (both in single-cast and multi-cast) streaming and real-time multimedia data.
The physical layers 105 and 110 include a set of rules that specifies the electrical and physical connection between the transmitter 101 and the receiver 106. At the device interfaces, the physical layers 105 and 110 specify the procedure for a correct transfer of data on slots, for example, TDMA/FDMA, encryption, interleaving, channel coding, FEC, and the reverse functions.
The data link layers 104 and 109 indicate how the transmitter 101 and the receiver 106 gains access to the medium specified in the physical layers 105 and 110. The data link layers 104 and 109 also define data formats, to include the framing of data within transmitted messages, error control procedures and other link control activities. From defining data formats to including procedures to correct transmission errors, the data link layers 104 and 109 are responsible for the reliable delivery of information. The data link layers 104 and 109 may be divided into two sublayers: a Logical Link Control (LLC) and a Media Access Control (MAC).
The transport layers 103 and 108 include an end-to-end real-time transport protocol (RTP)/real-time control protocol (RTCP) for providing standardized real-time feedback from the receiver 106 to the transmitter 101. One or more channels 111 and 112 may be used to transmit the control information. Both the RTP and the RTCP convey media data flows over a transmission control protocol (TCP) or a user datagram protocol (UDP). The RTP carries data with real-time requirements while the RTCP conveys information of the participants and monitors the quality of the RTP session. The transport layers 103 and 108 are responsible for guaranteeing that the transfer of information occurs correctly after a route has been established through the network 113. The transport layers 103 and 108 are used for error control, sequence checking, and other end-to-end data reliability factors.
The application layers 102 and 107 act as a window through which the applications gain access to all of the services provided by the underling protocols.
Data links in wireless networks by nature experience large variations in short term data rates due to changing channel and interference conditions. In packet networks supporting bursty data, network loading can also change rapidly. For many applications, buffering can be used to average out these variations. Slower rate adaptation can then be used, in conjunction with the buffering, to track out longer term changes in the channel rate.
However, buffering leads to delays which may not be permissible in certain interactive applications. That is, with tight delay constraints, short term drops in data rates results in dropped packets. In these cases, it is useful for the application to have fast feedback of the data communication losses so that the application can rapidly adjust to the lower rate and compensate for losses appropriately.
Two examples where such fast feedback is useful is (1) interactive or delay-sensitive video and (2) multi-player gaming video. Video can often be transmitted with a large range in quality by changing the spatial, temporal or pixel resolution. Feedback on the instantaneously channel rate can be used to adapt the video quality appropriately. Also, highly compressed video is typically transmitted with predictive coding to exploit temporal correlations. In predictive coding, frames at any one time instant are referenced against previous video frames. As a result, losses of video frames can propagate to several future frames until the next synchronization or intra-frame. Hence, fast feedback is useful to detect these losses quickly to reduce the error propagation.
In multi-player gaming video, communication losses result in state disconnect between different players. For example, the first player can think he has fired while the second player does not know he has been shot. In this example, fast detection of losses is needed to minimize the time delay in the discrepancies between the different player states.
As illustrated in the above examples, wireless channels can be unreliable and prone to errors and the end-to-end feedback from the wireless channel losses can be used at the application layers 102 and 107. Two existing mechanisms that can be used to provide feedback of the channel losses are (1) end-to-end feedback and (2) radio access technology feedback. First, communication protocols (such as RTP and RTCP) of the transport layers 103 and 108 provide the end-to-end feedback from the receiver 106 to the transmitter 101 and vice versa. RTCP packets contain direct information for quality of service (QoS) monitoring and congestion control of wireless channels. For example, sender reports (SR) and receiver reports (RR) exchange information on packet loss, jitter, and round-trip delay statistics of wireless channels. The transmitting end applications deliver SR to the receiving end applications and the receiving end applications deliver RR to the transmitting end applications.
The end-to-end feedback can be used by the transmitter 101 to adapt its channel rate to adjust to the channel errors. Also, the end-to-end feedback can be conducted completely at the transport layers 103 and 108 so the physical layers 105 and 110 are transparent to the applications. However, the end-to-end feedback has several drawbacks. For example, the end-to-end feedback has the cost of the round-trip end-to-end delay. Also, in wireless links, the end-to-end feedback consumes air-link resources, and generally only provides aggregate statistical information.
Second, some applications, for example cellular voice applications, are designed together with the radio access technology. This permits several cross-layer optimizations such as frame sizes that match the application, dedicated channels with appropriate rate adaptation, and physical layer specific feedback. More generally, given any radio access technology, one can develop a custom interface between a modem and a specific application. In this approach, however, the application interface has to be redesigned for each wireless technology. This eliminates the modularity between layers.
Therefore, it has been recognized by those skilled in the art that a need exists for feedback of channel losses that provides less delay and greater detail than the end-to-end feedback and can also be applied to a range of radio access technologies.